Media: Add two new intermediate image sizes, 1536px and 2048px. They …

…are meant to enhance the way WordPress displays images on the front-end on larger, high-density devices. They make it possible to generate more suitable `srcset` and `sizes` attributes, and not use the original, often non-optimized image.
Also change the default `max_srcset_image_width` value to match the new max size.
Props pierlo, azaozz.
See #43524.
git-svn-id: https://develop.svn.wordpress.org/trunk@46066 602fd350-edb4-49c9-b593-d223f7449a82